1. Field of the Invention
The present invention relates to an image projector, and more particularly to an image projector (such as a liquid crystal projector) for projecting an image from a reflection-type display panel (such as a reflection-type liquid crystal panel) onto a screen.
2. Description of the Prior Art
According to a known method for achieving appropriate illumination in an image projector of the type that projects an image displayed on a reflection-type display panel onto a screen, light for illumination is directed to the reflection-type display panel by the use of a reflection mirror that is disposed in the vicinity of an aperture stop so as to cover about a half of the aperture diameter. FIG. 12 shows the outline of the optical arrangement of such an image projector. This image projector has a projection optical system and an illumination optical system. The projection optical system is composed of a front lens unit (GrF), a reflection mirror (M), an aperture stop (A), and a rear lens unit (GrR). The illumination optical system is composed of a condenser lens (CL).
The light beam from a light source (1) is formed into a substantially parallel beam by a reflector (2), and is then condensed by the condenser lens (CL) so as to form an image of the light source. The light source (1), the reflector (2), the condenser lens (CL), and the reflection mirror (M) are so arranged that the image of the light source is formed at the position of the aperture stop (A). Thus, this optical arrangement conforms to that of the so-called Koehler illumination. In FIG. 12, the light source image is shown as converging on a single point, but in reality it is formed as an image having certain dimensions. The light beam reflected from the reflection mirror (M) passes through the rear lens unit (GrR), and then enters a color separating/integrating prism (Pr1), where the light beam is separated into a plurality of light beams of different colors so as to illuminate the display surfaces of a plurality of reflection-type display panels (P) individually, with the light beam of each color illuminating the entire display surface of the corresponding display panel. It should be noted that, in FIG. 12, only one display panel (P) for one of the colors used is illustrated.
Since the reflection-type display panel (P) employs a reflection-type liquid crystal panel, the light beam that illuminates the display panel (P) is reflected according to the pattern formed by the pixels of the display panel (P). The light beam reflected from the display panel (P) is then, by the color separating/integrating prism (Pr1), formed into a single light beam to be projected (hereafter referred to as the "projection light beam"), which then passes through the rear lens unit (GrR). Thereafter, the projection light beam passes, at the position of the aperture stop (A) where the reflection mirror (M) is disposed, through the other half of the aperture stop that is not covered by the reflection mirror (M). Thus, the projection light beam that has passed through the aperture stop (A) without striking the reflection mirror (M) forms, through the front lens unit (GrF), a display image on the screen (S).
In a case where a high-power light source such as a metal halide lamp is employed as the light source (1) in order to secure sufficiently bright illumination in the image projector, it is customary to use an integrator, because it helps prevent uneven illumination as may be caused by such a light source, and make the most of the light beam emitted from the light source (1) for the illumination of the surface of the display panel (P). For example, Japanese Laid-Open Patent Applications No. H9-243990 and others propose image projectors that are based on the above-described illumination method (FIG. 12) and that additionally include an integrator. The integrator employed in these examples is of the type that has two lens arrays of which each is composed of an array (i.e. rows and columns) of lens elements.
The recent trend toward more compact display panels has been accompanied by the trend toward more compact projection optical systems. As a natural consequence of this trend, projection optical systems have come to have smaller and smaller aperture diameters. On the other hand, if a high-power light source is employed in a projector to obtain brighter illumination, it is inevitable to use a reflector having an accordingly large diameter. In cases where the aperture diameter is small and the reflector diameter is large, it is difficult to realize an optical arrangement based on the illumination method proposed by Japanese Laid-Open Patent Application No. H9-243990, because of the excessively great difference between the diameter of the second lens array, which has to be disposed at the position of the aperture stop (A), and that of the first lens array, which has to be disposed immediately behind the reflector (2). In particular, it is extremely difficult to realize such an optical arrangement in a projection television system of a backward-projection type (rear type), where the focal length of the projection optical system tends to be short and thus the aperture diameter tends to be small.